Goodpasture (GP) disease is an autoimmune disorder described only in humans. In GP patients, autoantibodies against the non-collagenous C-terminal domain (NC1) of the type IV collagen α3 chain (“Goodpasture antigen”) cause a rapidly progressive glomerulonephritis and often lung hemorrhage, the two cardinal clinical manifestations of the GP syndrome (see 1 for review. The reference numbers in this section correspond to reference list of Example 1).
The idea that common pathogenic events exist at least for some autoimmune disorders is suggested by the significant number of patients displaying more than one autoimmune disease, and also by the strong and common linkage that some of these diseases show to specific MHC haplotypes (31, 32). The experimental observation that the autoantigen is the leading moiety in autoimmunity and that a limited number of self-components are autoantigenic (31), suggest that these self-components share biological features with important consequences in self/non-self recognition by the immune system. One possibility is that triggering events, by altering different but specific self-components, would result in abnormal antigen processing. In certain individuals expressing a particular MHC specificity, the abnormal peptides could be recognized by non-tolerized T cells and trigger an immune response (1).
We have previously explored the GP antigen to identify biological features of relevance in autoimmune pathogenesis. Since the NC1 domain is a highly conserved domain among species and between the different type IV collagen α chains (α1–α6) (2), the exclusive involvement of the human α3(IV)NC1 in a natural autoimmune response suggests that this domain has structural and/or biological peculiarities of pathogenic relevance. Consistent with this, the N-terminus of the human antigen is highly divergent, and it contains a unique five-residue motif (KRGDS9) that conforms to a functional phosphorylation site for type A protein kinases (3, 4). Furthermore, the human α3 gene, but not the other related human or homologous genes from other species, is alternatively spliced and generates multiple transcripts also containing the phosphorylatable N-terminal region (5–7). Recent studies indicate that the phosphorylation of the N-terminus of the GP antigen by cAMP-dependent protein kinase is up regulated by the presence of the alternative products (see Example 3 below). Specific serine phosphorylation and pre-mRNA alternative splicing are also associated with the biology of other autoantigens including the acetylcholine receptor and myelin basic protein (MBP) (4). The latter is suspected to be the major antigen in multiple sclerosis (MS), another exclusively human autoimmune disease in which the immune system targets the white matter of the central nervous system. GP disease and MS are human disorders that display a strong association with the same HLA class II haplotype (HLA DRB1*1501)(32, 33). This, along with the recent report of death by GP disease of an MS patient carrying this HLA specificity (34), supports the existence of common pathogenic events in these human disorders.
Thus, specific serine/threonine phosphorylation may be a major biological difference between the human GP antigen, the GP antigens of other species, and the homologous domains from the other human α(IV) chains, and might be important in pathogenesis (1, 4).
Therefore, the identification and isolation of the specific serine/threonine kinase that phosphorylates the N-terminal region of the human GP antigen would be very advantageous for the diagnosis and treatment of GP syndrome, and possibly for other autoimmune disorders.